SPRING1-C-20MAR02-MT-FRL: The birds and bees are out doing their job on the first day of spring in Golden Gate Park. Chronicle photo by Frederic Larson ALSO RAN 09/12/02 CAT

Photo: Frederic Larson

SPRING1-C-20MAR02-MT-FRL: The birds and bees are out doing their...

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"Bride of RoboFly" is Cal Tech scientists' whimsical name for their robotic model of a generic insect that can be used to model the wing-flapping and other flight dynamics of a variety of different insects. The descendant of an earlier robot called "RoboFly," Bride of RoboFly's wingspan stretches about two feet from wingtip to wingtip. The robot contains several motors and can be remotely operated while floating inside a large tank of mineral oil. Photo: Courtesy of Michael Dickinson/California Institute of Technology

Photo: Courtesy Of Michael Dickinson/Ca

"Bride of RoboFly" is Cal Tech scientists' whimsical name for their...

Insects were the world's first aviators, and to this day their evolutionary descendants perform aerial stunts more dashing than the Blue Angels: They zip past your eyes like meteors, then hover like helicopters over flowers, then vanish out of sight before you can swat them.

Scientifically speaking, insect flight was shrouded in mystery for much of the 20th century and even now is haunted by enigmas.

Studies have shown how insects fly by frantically flapping their wings and taking advantage of physical forces too microscopic to be exploited by airplanes. Now scientists are beginning to investigate how insects' brains, although extremely tiny, can manage the incredibly complex motions required for them to stay aloft.

For example, there's an urban legend that many decades ago, scientists analyzed the plump bodies and stubby wings of bumblebees and concluded they were too heavy to fly. Over the years, during repeated retellings of this story in schoolyards and barrooms, it acquired a punch line: "But bees don't know they can't fly, so they fly anyway."

The urban legend is based on fact: A bumblebee study was conducted in 1934 by the European scientists Antoine Magnan and Andre Saint-Lague. They applied mathematical analysis and known principles of flight to calculate that bee flight was "impossible," say insect-flight researchers Douglas L. Altshuler, Michael Dickinson and three colleagues at Caltech and the University of Nevada, Las Vegas in an article for today's issue of the Proceedings of the National Academy of Sciences.

"Since this time," the authors note, "bees have symbolized both the inadequacy of aerodynamic theory as applied to animals and the hubris with which theoreticians analyze the natural world."

The mystery of bee flight is the tip of the iceberg, though. Researchers have long struggled to understand the flight of all types of insects, from teeny fruit flies to the satanic-looking dragonflies. That's partly because insect aviation and human aviation are very different feats; the physics of the latter can't explain the physics of the former, as scientists have long known. Because of their tiny size, flying creatures like bumblebees, dragonflies, fruit flies and other insects must take into account microscopic and incredibly complex physical forces and effects that have negligible impact on 747s.

The latest example of such research is the study by Altshuler, Dickinson and their colleagues. As they report in their article, they used high-speed (6,000 frames per second) digital cameras to image the wing-flapping of honeybees leaving a hive at the University of Nevada, Las Vegas. The scientists also analyzed bee motions inside transparent acrylic chambers, where the insects made a beeline to containers of sugary fluid and pollen grains.

Their conclusions include that bees, while hovering, swing their wings over amplitudes of about 90 degrees, a narrower range than other insects. But they also beat their wings unusually quickly for insects their size. Insect-flight experts have long assumed that the smaller the insect, the faster it beats its wings; but in the case of honeybees, the creature -- technically known as Apis mellifera -- beats its 10mm-wide wings about 240 times per second, faster than the much smaller fruit fly, which manages only 200 beats per second.

In addition, the researchers observed how the creatures flew under stressful, high-altitude conditions when they were flying inside chambers containing a low-density mixture of oxygen and helium gases. True to the saying "busy as a bee," the bees put in a hard day's work for the scientists, who, as Altshuler's article notes, continued analyzing the little creatures until they "exhibited lethargy or disinterest."

Mathematician Laura A. Miller of the University of Utah, who works on mathematical models of insect flight, said the Altshuler team's article is "excellent ... a significant contribution to the field of insect flight aerodynamics ... (It) should motivate many future studies on comparative insect flight."

Today's paper is the latest in a series of studies on insect flight over the last decade. A key finding has been that there's a big difference between the flight of insects and the flight of airplanes.

An airplane flies because the upper part of its wing is a fixed, curved structure. That way, air flowing over the top of the wing has to travel faster, and a greater distance, in the same amount of time as air flowing under the wing. This causes the upper wing's air pressure to drop, so that the higher pressure beneath the wing forces the wing upwards -- and the plane with it. That's the basic principle behind airplane flight.

For insects, flight is much more complicated. In insects, "the morphology (shape) of the wing has almost no role," Dickinson, a professor of bioengineering at the California Institute of Technology, said in an interview. "What matters is not the shape of the wing but how the insect moves it. That's very different from conventional (airplane) aerodynamics, where the shape of the wing is everything."

Insect wings are constantly in motion, he said, so they're more like propellers than fixed aircraft wings.

In the 1990s, crucial work in the field of insect-flight research was conducted by Charles Ellington of Cambridge University in England. He and other scientists, including Dickinson, built big "robotic" models of insects. With these mechanical critters -- "Robofly," Dickinson named one of them -- they measured the forces on different parts of the robots' wings as they flapped back and forth. Also, improved observational techniques (using miniature wind tunnels) and high-speed computers made it possible to model the dynamics of air around the flapping wings. Also in the 1990s, experimenters using sensitive observational equipment and high-speed cameras discovered that a beating insect wing forms a swirling funnel of air -- technically known as the leading-edge vortex, a kind of micro-tornado -- just above, and clinging to, the upper part of the wing. Air pressure inside the vortex is lower than surrounding air, just as air pressure inside a tornado is lower than in surrounding air. Thus higher-pressure air beneath the bug wing pushes it upward, providing lift to the insect.

But such things alone don't explain how insects stay aloft once they're airborne.

Bugs' wings also flap backward and curl while flapping. This rotational motion creates additional uplift for the same basic reason that the backspin on a soaring baseball keeps it aloft longer than it would in the absence of backspin. To be specific: Because the ball's top turns back toward the pitcher while the bottom turns away from him, air flows faster over the top than the bottom. Faster-flowing air has lower pressure. Therefore, the air pressure is lower on top of the ball, hence the higher pressure underneath the ball pushes it upward. This gives the ball "lift," which keeps it from falling back to Earth as fast as it would in the absence of backspin.

Scientists still have only scratched the surface of the puzzle of insect flight. An insect must continually flap its wings to stay aloft, and must continually alter its wing and body orientations to counteract the numerous forces that are dragging it downward. This requires more than wing agility; it also requires a sharp little brain.

"What would you need to know if you really wanted to build a fly?" Dickinson asks. "Understanding what the wings do is just a tiny part of it." If you built a robotic fly and its wing simply flapped back and forth, "the thing very rapidly crashes like a brick."

To fly, "every moment (the insect) has to be constantly figuring out: 'Am I yawing? Am I pitching? Am I rolling? Am I drifting backward? Am I falling? Am I rising? And all that information is constantly streaming into a brain the size of about a poppy seed. Understanding insect flight requires understanding how that little 'computer' works -- and that's just as essential as understanding how the wings work.

"There's still a lot of stuff to be excited about -- we're not going to solve it all in my lifetime."